I've seen the AI build forts even before the latest patch. And they've attacked my forts too!
Personally I think forts can be very useful, but also expensive. If I'm really settling my border and want to defend it well, I'll usually place a few forts at key locations. Then any attacker must either attack the fort (delaying his attack so I can prepare an army), or he can ignore it, risking being trapped between two armies.
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